Physical Activity During Pregnancy - Past and Present

Abstract The possible implications of physical activity during the period of pregnancy have been much debated over recent decades. This brief appraisal integrates knowledge from an array of position papers, systematic reviews, meta-analyses, and recommendations provided by specialty board committees. The medical community is becoming more and more aware of the beneficial effects of mild and moderate physical activity on the mother and the fetus, including improved clinical correlates of subsequent vaginal delivery, as contrasted to the clearly unbeneficial effects of a sedentary lifestyle.


INTRODUCTION
Pregnancy is associated with considerable physiological and psychological changes that may promote sedentary behavior in women. Such behavior is associated with increased risks for gestational diabetes, arterial hypertension and excessive weight gain, all of them jeopardizing maternal and fetal well-being in pregnancy.
Since at present a large proportion of pregnant women engage in regular physical activity during pregnancy, it is imperative that the obstetrician becomes familiar with the e ects that physical activity can exert on maternal health and pregnancy outcomes. e debate with regard to the possible risks and bene ts of physical activity during pregnancy goes back to biblical times when Hebrew writers observed that slaves had much easier deliveries than their Egyptian patrons who led a sedentary life [1]. Both Aristotle (4 th Century B.C.E.) and later Plutarch of Chaeronea (1 st /2 nd Century C.E.) urged Spartan and other Greek women to exercise in order to decrease the pain of childbearing [2].
In contrast, it is amazing that by the end of the 19 th century pregnant women were encouraged to remain indoors, keep themselves "con ned", and advised to avoid exercise since "during gymnastics there was a risk that the female organs might slip when straddling and female fertility may be at severe risk" [2].
During the 20 th century, the advances in knowledge of anatomy and physiology led to a dramatic change in the approach to physical activity in general and during pregnancy in particular. Already in the 1930's antenatal exercise classes taught by Heardman, a physiotherapist, were introduced in England and Sweden [3]. In 1949, the U.S. Children's Bureau published their recommendation for prenatal exercise stating that, in cases without maternal complications, pregnant women may continue their typical activities, such as home duties or gardening on a daily basis. In 1959, Karmel and Bing, also in the United States, popularized the Lamaze method that focused on physiological and psychological preparation for childbirth [4]. e Lamaze method is still very popular in many medical centers today.
In 2015, the American College of Obstetricians and Gynecologists published a Committee Opinion with regard to physical activity and exercise during pregnancy and the postpartum period. e Committee recommended that pregnant women with no risk factors should exercise 20-30 minutes per day engaging in aerobic and strengthconditioning exercise before, during, and a er pregnancy [5].
Overall, the last 100 years witnessed a great interest in this subject as evidenced by an enormous bulk of literature published in the most prestigious medical journals. Below, we present a personal selection of some of these reports. Miriam Katz, MD the contraction of skeletal muscle that increases energy expenditure above a basal level". In contrast, "exercise" represents "a form of physical activity that is planned, structured, repetitive, and performed with the goal of improving health or tness" [6]. e results of a detailed literature review on the physiology of exercise in pregnancy were, among others, that [7]: -maximal aerobic power is well-preserved in pregnant women who remain physically active; -in contrast, anaerobic working capacity may be reduced in late gestation; -responses to prolonged submaximal exercise (>30 minutes) in late gestation include a moderate reduction in maternal blood glucose concentration, which may transiently reduce fetal glucose availability; -the normal response to sustained submaximal (maternal) exercise is an increase in fetal heart rate (FHR) baseline; -transient reductions in FHR reactivity, fetal breathing movements, and FHR variability may also occur in association with more strenuous exercise. Consequently, Wolfe and Weissgerber stated that healthy women undergoing normal pregnancies can participate safely in moderate prenatal tness programs and maintain or improve physical tness without harming fetal growth and development [7].

PHYSIOLOGICAL ASPECTS
During pregnancy, sustained exercise sessions cause an intermittent reduction in oxygen and substrate delivery to the interphase that may exceed 50% during the exercise, however, regular bouts of sustained exercise or exercise training probably improve oxygen and substrate delivery to the fetus at rest [8].
As to work e ciency, the energy cost of standard submaximal exercise in pregnancy depends on whether the activity is weight-bearing or weight-supported. For weight-supported exercise (such as cycling), the energy requirement above resting metabolic rate is not signi cantly a ected by pregnancy or advancing gestational age. In contrast, for weight-bearing activities (such as walking or jogging), the energy requirement increases in proportion to maternal weight gain [7].
Swimming has been advocated as an optimal exercise during pregnancy due to its buoyant e ects and the thermally conductive properties of water [9]. Stationary upright cycling for 30 minutes at a maternal heart rate around 140 beats per minute (bpm) or exercising for 15 minutes at a rate of 155 bpm has no negative e ect on the woman or the fetus. Also walking is a recommended activity. In contrast, extreme sports such as scuba diving, high-altitude exercise including skiing, hiking, mountain biking, and running, as well as sports with abdominal trauma risk from contact or falling are discouraged. e list of the latter activities also includes downhill skiing, waterskiing, horseback riding, road cycling, sur ng, basketball, racquet sports, ice hockey, soccer, and gymnastics [6,9,10]. De nitely, both prudence and moderation are a good general advice. Interestingly, a recent careful testing of maternal and fetal responses to 26 yoga postures con rmed that yoga is very safe not only for the mother, but also for the baby [11]. Antenatal pelvic oor muscle exercises are gaining in popularity and there are essentially no harmful e ects ascribed to them, either, whereas they may prevent urinary incontinence in late pregnancy and de nitely prevent urinary incontinence in the postnatal period [12].
From a systematic review, the exercise intensity prescription should be mild to moderate for previously sedentary women, and moderate to high for previously active women [10]. us, moderate intensity activity can always be a good advice of the obstetrician for gravidae without contraindications to exercise. ese recommendations are based on the current knowledge of the impact of moderateintensity, low-impact, aerobic exercise performed by the pregnant woman at least three times a week and thus they may be subject to broadening, rather than narrowing, as further evidence accumulates. Let us cite here the Jukic et al. data [13] where it was shown in a prospective study that vigorous recreational activity does not appear to be detrimental to the timing of birth or to birthweight. In line, in a carefully controlled study, inactive, regularly active, and highly active pregnant women were subjected to one or two 30-minute treadmill sessions. Umbilical artery Doppler indices were similar pre-exercise and postexercise among groups. Moreover, postexercise FHR tracings met the criteria for reactivity within 20 minutes a er all the tests [14], consistent with the lack of acute aggravation in fetal well-being.
Absolute and relative medical contraindications to exercise during pregnancy are given in Table I. Overall, any signi cant obstetric or other medical conditions should prevent from engagement into unsupervised vivid physical activity. As evidenced in a survey, approximately half of the American obstetricians recommended a reduction in exercise load during the third trimester [15], possibly as a preventive measure.

CLINICAL ASPECTS
Since it seems possible to improve pregnancy outcomes in both healthy, low-risk women and a variety of high-risk populaces by simply modifying maternal physical activity and dietary carbohydrate intake during pregnancy [8], this notion sparked an increased interest in maternal exercises. e goals of physical activity during pregnancy include maintaining maternal well-being and establishing a pattern of regular activity that will ultimately prevent the onset of chronic disease associated with a sedentary lifestyle. e main bene ts of physical activity in pregnancy are as follows: it helps control weight gain, improves blood circulation (including less swelling), reduces the risks of developing preeclampsia and gestational diabetes, and increases the chances for a shorter spontaneous vaginal delivery. As for the baby, physical activity normalizes maternal blood glucose and reduces insulin resistance, thus a ecting fetal size. Moreover, placental blood ow and nutrient delivery to the developing fetus become stabilized [8,16]. e majority of women who gained excessive weight are at risk of developing gestational diabetes and preeclampsia, and of having macrosomic babies, all of which can lead to increased complications in labor and delivery. Furthermore, these big babies born to overweight mothers are at a signi cant risk for developing type 2 diabetes later in life. us, it is of importance that in their robust meta-analysis which included 442 studies addressing the e ect of exercise on pregnancy, Tobias et al. [17] concluded that exercise signi cantly reduces the odds for the development of gestational diabetes. A review of suggested recommendations for prevention and treatment of gestational diabetes by means of exercise has recently been published [18]. In a Canadian prospective cohort study, physical activity in the rst half of pregnancy signi cantly decreased the occurrence of fetal macrosomia without a ecting preterm birth or low birth weight [19].
Other pregnancy-related complications which may seriously jeopardize the pregnancy outcome are hypertensive disorders. ey a ect about 3-9% of all pregnancies worldwide, with preeclampsia diagnosed in 2-4% of them. Even today hypertensive disorders are one of the main causes of maternal and fetal morbidity and mortality [20]. erefore, it is important to note that physical activity prior to and during pregnancy has been found to protect the gravidae and parturients from hypertensive disorders by controlling their weight gain, lowering the blood pressure, improving maternal lipid pro le, reducing the oxidative stress and in ammation, as well as improving placental growth and vasculature [8]. A group of Scandinavian researchers reported a signi cantly lower incidence of elevated blood pressure in a group of 59 573 prospective mothers who exercised more than 25 times a month starting from 16 weeks of pregnancy [21].
Regarding other central clinical correlates, one of the main fears of the pregnant patient is that physical activity may increase her risk for miscarriage or premature delivery. Most studies addressing this question state that in a patient with no risk factors (i.e., no previous premature labor, cervical incompetence, etc.) physical activity is safe. However, one should advise against exertion (high-intensity and long-duration exercise) in early pregnancy, especially at 8-12 weeks. In contrast, some already cited data support a reduced risk of preterm birth with vigorous recreational activity, particularly with increased frequency of recreational type activity [13]. In a large Dutch-British population-based cohort study on daily physical activity, 11 759 singleton pregnancies were assessed and no association between physical e ort and duration of gestation or fetal survival was found [22].
In a retrospective study, Bungum et al. [23] found that nulliparous women exercising during pregnancy had substantially increased odds for a normal vaginal delivery and that such subjects had a lower chance of Cesarean section delivery. Impressively, for those sedentary during pregnancy, the odds ratio for Cesarean delivery was found to be as high as 4.48 [23].
e results of a meta-analysis of observational studies suggested an inverted U-shaped association between physical activity and birth weight [24]. In other words, both extremes in physical activity of the pregnant woman are unbene cial.  ere are undoubtedly many advantages of exercise during pregnancy and therefore, a er excluding medical and pregnancy-related risk factors, obstetricians should strongly recommend to their patients to be physically active. Non-extreme physical activity prior to and during pregnancy is not only safe for fetal health but is also associated with numerous maternal health bene ts, including prevention and better control of gestational diabetes, excessive weight gain, and increased blood pressure. It is worth mentioning that exercise also reduces lower back pain and has positive e ects on both maternal mental health and quality of life. CONCLUSIONS e recent recommendations from the American College of Obstetricians and Gynecologists clearly embrace the research cited above. ey can be summarized as follows [5]: 1. physical activity in pregnancy has minimal risks and has been shown to bene t most women, although some modi cation to exercise routines may be necessary because of normal anatomic and physiologic changes and fetal requirements; 2. a thorough clinical evaluation is necessary before recommending an exercise program to ensure that a patient does not have medical reasons to avoid exercise; 3. women with uncomplicated pregnancies should be encouraged to engage in both aerobic and strengthconditioning exercises before, during, and a er pregnancy; 4. obstetrician-gynecologists and other obstetric care providers should carefully evaluate women with medical or obstetric complications before making recommendations on their physical activity participation during pregnancy; 5. regular physical activity during pregnancy improves or maintains physical tness, helps with weight management, reduces the risk of gestational diabetes in obese women, and enhances psychological well-being. is statement testi es to a tremendous shi in both medical thinking and practical approach from the necessarily conservative position of early guidelines because of lack of research decades ago to the current active encouragement of pregnant women to do mild and moderate exercise as a physiological means to maintain a healthy, or healthier, pregnancy. What a spectacular change we are witnessing!